High-strength steel sheet having superior workability and manufacturing method therefor

ABSTRACT

Provided is a steel sheet and a method for manufacturing same, the steel sheet, which is optimized a composition and microstructure, and which can be used for automobile parts and the like, having superb bendability, and superior balance of strength and ductility and of strength and hole expansion ratio.

TECHNICAL FIELD

The present invention relates to a steel sheet that may be used forautomobile parts and the like, and to a steel sheet having high strengthcharacteristics and superior workability and a manufacturing methodtherefor.

BACKGROUND ART

In recent years, the automobile industry is paying attention to ways toreduce material weight and secure occupant stability in order to protectthe global environment. In order to meet these requirements forstability and weight reduction, the application of a high strength steelsheet is rapidly increasing. In general, it has been known that as thestrength of the steel sheet increases, the workability of the steelsheet decreases. Therefore, in the steel sheet for automobile parts, asteel sheet having superior workability represented by ductility,bendability, and hole expandability while having high strengthcharacteristics is required.

As a technique for improving workability of a steel sheet, a method ofutilizing tempered martensite is disclosed in Patent Documents 1 and 2.Since the tempered martensite made by tempering hard martensite issoftened martensite, there is a difference in strength between thetempered martensite and the existing untempered martensite (freshmartensite). Therefore, when fresh martensite is suppressed and thetempered martensite is formed, the workability may be increased.

However, by the techniques disclosed in Patent Documents 1 and 2, abalance (TS×E1) of tensile strength and elongation does not satisfy22,000 MPa % or more, meaning that it is difficult to secure a steelsheet having superior strength and ductility.

Meanwhile, transformation induced plasticity (TRIP) steel usingtransformation-induced plasticity of retained austenite was developed inorder to obtain both high strength and superior workability forautomobile member steel sheets. Patent Document 3 discloses TRIP steelhaving superior strength and workability.

Patent Document 3 discloses improving high ductility and workability byincluding polygonal ferrite, retained austenite, and martensite, but itcan be seen that Patent Document 3 uses bainite as a main phase, andthus, the high strength is not secured and the balance (TS×E1) of thetensile strength and elongation also does not satisfy 22,000 MPa % ormore.

That is, the demand for a steel sheet having superior workability, suchas ductility, bendability, and hole expandability while having highstrength, is not satisfied.

RELATED ART DOCUMENT

-   (Patent Document 1) Korean Patent Laid-Open Publication No.    10-2006-0118602-   (Patent Document 2) Japanese Patent Laid-Open Publication No.    2009-019258-   (Patent Document 3) Korean Patent Laid-Open Publication No.    10-2014-0012167

DISCLOSURE Technical Problem

The present invention provides a high-strength steel sheet havingsuperior ductility, bendability, and hole expandability by optimizing acomposition and microstructure of the steel sheet and a method formanufacturing the same.

An object of the present invention is not limited to the abovementionedcontents. Additional problems of the present invention are described inthe overall content of the specification, and those of ordinary skill inthe art to which the present invention pertains will have no difficultyin understanding the additional problems of the present invention fromthe contents described in the specification of the present invention.

Technical Solution

In an aspect of the present invention, a high-strength steel sheethaving superior workability according may include: by wt %, C: 0.25 to0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 5.0% or less, P: 0.15% orless, S: 0.03% or less, N: 0.03% or less, a balance of Fe, andunavoidable impurities, and includes, as microstructures, ferrite whichis a soft structure, and tempered martensite, bainite, and retainedaustenite which are hard structures, and may satisfy the following[Relational Expression 1].

0.4≤[H]_(F)/[H]_(TM+B+γ)≤0.9  [Relational Expression 1]

In the above Relational Expression 1, [H]_(F) and [H]_(TM+B+γ) arenanohardness values measured using a nanoindenter, [H]_(F) is an averagenanohardness value Hv of the ferrite which is the soft structure,[H]_(TM+B+γ) is the average nanohardness value Hv of the temperedmartensite, the bainite, and the retained austenite which are the hardstructures.

The steel sheet may further include one or more of the following (1) to(9).

(1) one or more of Ti: 0 to 0.5%, Nb: 0 to 0.5%, and V: 0 to 0.5%

(2) one or more of Cr: 0 to 3.0% and Mo: 0 to 3.0%

(3) one or more of Cu: 0 to 4.5% and Ni: 0 to 4.5%

(4) B: 0 to 0.005%

(5) one or more of Ca: 0 to 0.05%, REM: 0 to 0.05% excluding Y, and Mg:0 to 0.05%

(6) one or more of W: 0 to 0.5% and Zr: 0 to 0.5%

(7) one or more of Sb: 0 to 0.5% and Sn: 0 to 0.5%

(8) one or more of Y: 0 to 0.2% and Hf: 0 to 0.2%

(9) Co: 0 to 1.5%

A total content (Si+Al) of Si and Al may be 1.0 to 6.0 wt %

The microstructure of the steel sheet includes, by volume fraction, 30to 70% of tempered martensite, 10 to 45% of bainite, 10 to 40% ofretained austenite, 3 to 20% of ferrite, and unavoidable microstructurecomponents.

A balance B_(T·E) of tensile strength and elongation expressed by thefollowing [Relational Expression 2] is 22,000 (MPa %) or more, a balanceB_(T·H) of tensile strength and a hole expansion ratio expressed by thefollowing [Relational Expression 3] is 7*10⁶ (MPa²%^(1/2)) or more, andbendability B_(R) expressed by the following [Relational Expression 4]may satisfy a range of 0.5 to 3.0.

B _(T·E)=[Tensile Strength(TS,MPa)]*[Elongation(El,%)]  [RelationalExpression 2]

B _(T·H)=[Tensile Strength(TS,MPa)]²*[Hole ExpansionRatio(HER,%)]^(1/2)  [Relational Expression 3]

B _(R) =R/t  [Relational Expression 4]

In the above Relational Expression 4, R is a minimum bending radius (mm)at which cracks do not occur after a 90° bending test, and t is athickness (mm) of the steel sheet.

In another aspect of the present invention, a manufacturing method forhigh-strength steel sheet having superior workability may include:heating and hot rolling a steel slab including, by wt %, C: 0.25 to0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 5.0% or less, P: 0.15% orless, S: 0.03% or less, N: 0.03% or less, a balance of Fe, andunavoidable impurities; coiling the hot-rolled steel sheet; performing ahot-rolled annealing heat treatment on the coiled steel sheet in atemperature within a range of 650 to 850° C. for 600 to 1700 seconds;cold rolling the hot-rolled annealing heat-treated steel sheet; heating(primary heating) the cold-rolled steel sheet to a temperature within arange of Ac1 or higher and less than Ac3, and maintaining (primarymaintaining) the cold-rolled steel sheet for 50 seconds or more; cooling(primary cooling) the primarily heated steel sheet to a temperaturewithin a range of 100 to 300° C. at an average cooling rate of 1° C./sor more; heating (secondary heating) the primarily cooled steel sheet toa temperature within a range of 300 to 500° C. at an average temperatureincrease rate of 5° C./s or more, and maintaining (secondarilymaintaining) the primarily cooled steel sheet for 50 seconds or more;and cooling (secondarily cooling) the secondarily heated steel sheet toroom temperature.

The steel slab may further include one or more of the following (1) to(9).

(1) one or more of Ti: 0 to 0.5%, Nb: 0 to 0.5%, and V: 0 to 0.5%

(2) one or more of Cr: 0 to 3.0% and Mo: 0 to 3.0%

(3) one or more of Cu: 0 to 4.5% and Ni: 0 to 4.5%

(4) B: 0 to 0.005%

(5) one or more of Ca: 0 to 0.05%, REM: 0 to 0.05% excluding Y, and Mg:0 to 0.05%

(6) one or more of W: 0 to 0.5% and Zr: 0 to 0.5%

(7) one or more of Sb: 0 to 0.5% and Sn: 0 to 0.5%

(8) one or more of Y: 0 to 0.2% and Hf: 0 to 0.2%

(9) Co: 0 to 1.5%

A total content (Si+Al) of Si and Al included in the steel slab may be1.0 to 6.0 wt %.

The steel slab may be heated to a temperature within a range of 1000 to1350° C., and may be subjected to finish hot rolling in a temperaturewithin a range of 800 to 1000° C.

The hot-rolled steel sheet may be coiled at a temperature within a rangeof 300 to 600° C.

A reduction ratio of the cold rolling may be 30 to 90%.

The cooling rate of the secondary cooling may be 1° C./s.

Advantageous Effects

According to an aspect of the present disclosure, it is possible toprovide a steel sheet particularly suitable for automobile parts becausethe steel sheet has superior strength as well as superior workabilitysuch as ductility, bending workability, and hole expansion ratio.

BEST MODE

The present invention relates to a high strength steel sheet havingsuperior workability and a method for manufacturing the same, andexemplary embodiments in the present invention will hereinafter bedescribed. Exemplary embodiments in the present invention may bemodified into several forms, and it is not to be interpreted that thescope of the present invention is limited to exemplary embodimentsdescribed below. The present exemplary embodiments are provided in orderto further describe the present invention in detail to those skilled inthe art to which the present invention pertains.

The inventors of the present invention recognized that, in atransformation induced plasticity (TRIP) steel including bainite,tempered martensite, retained austenite, and ferrite, when controlling aratio of specific components included in the retained austenite and theferrite to a certain range while promoting stabilization of the retainedaustenite, it is possible to simultaneously secure workability andstrength of a steel sheet by reducing a difference in hardness betweenphases of the retained austenite and the ferrite. Based thereon, thepresent inventors have reached the present invention by devising amethod capable of improving ductility and workability of the highstrength steel sheet.

Hereinafter, a high strength steel sheet having superior workabilityaccording to an aspect of the present invention will be described inmore detail.

To an aspect of the present invention, a high-strength steel sheethaving superior workability according may include: by wt %, C: 0.25 to0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 5.0% or less, P: 0.15% orless, S: 0.03% or less, N: 0.03% or less, a balance of Fe, andunavoidable impurities, and includes, as microstructures, ferrite whichis a soft structure, and tempered martensite, bainite, and retainedaustenite which are hard structures, and may satisfy the following[Relational Expression 1].

0.4≤[H]_(F)/[H]_(TM+B+γ)≤0.9  [Relational Expression 1]

In the above Relational Expression 1, [H]_(F) and [H]_(TM+B+γ) arenanohardness values measured using a nanoindenter, [H]_(F) is an averagenanohardness value Hv of the ferrite which is the soft structure,[H]_(TM+B+γ) is the average nanohardness value Hv of the temperedmartensite, the bainite, and the retained austenite which are the hardstructures.

Hereinafter, compositions of steel according to the present inventionwill be described in more detail. Hereinafter, unless otherwiseindicated, % indicating a content of each element is based on weight.

The high strength steel sheet having superior workability according toan aspect of the present invention includes, by weight, C: 0.25 to0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 5.0% or less, P: 0.15% orless, S: 0.03% or less, N: 0.03% or less, a balance of Fe, andunavoidable impurities. In addition, the high strength steel sheet mayfurther include one or more of Ti: 0.5% or less (including 0%), Nb: 0.5%or less (including 0%), V: 0.5% or less (including 0%), Cr: 3.0% or less(including 0%), Mo: 3.0% or less (including 0%), Cu: 4.5% or less(including 0%), Ni: 4.5% or less (including 0%), B: 0.005% or less(including 0%), Ca: 0.05% or less (including 0%), REM: 0.05% or less(including 0%) excluding Y, Mg: 0.05% or less (including 0%), W: 0.5% orless (including 0%), Zr: 0.5% or less (including 0%), Sb: 0.5% or less(including 0%), Sn: 0.5% or less (including 0%), Y: 0.2% or less(including 0%), Hf: 0.2% or less (including 0%), Co: 1.5% or less(including 0%). In addition, a total content (Si+Al) of Si and Al may be1.0 to 6.0%.

Carbon (C): 0.25 to 0.75%

Carbon (C) is an essential element for securing strength of a steelsheet, and is also an element for stabilizing the retained austenitethat contributes to the improvement in ductility of the steel sheet.Accordingly, the present invention may include 0.25% or more of carbon(C) to achieve such an effect. A preferable content of carbon (C) mayexceed 0.25%, may be 0.27% or more, and may be 0.30% or more. The morepreferable content of carbon (C) may be 0.31% or more. On the otherhand, when the content of carbon (C) exceeds a certain level, coldrolling may become difficult due to an excessive increase in strength.Therefore, an upper limit of the content of carbon (C) of the presentdisclosure may be limited to 0.75%. The content of carbon (C) may be0.70% or less, and the more preferable content of carbon (C) may be0.67% or less.

Silicon (Si): 4.0% or less (excluding 0%)

Silicon (Si) is an element that contributes to improvement in strengthby solid solution strengthening, and is also an element that improvesworkability by strengthening ferrite and homogenizing a structure. Inaddition, silicon (Si) is an element contributing to a generation of theretained austenite by suppressing precipitation of cementite. Therefore,in the present invention, silicon (Si) may be necessarily added toachieve such an effect. The preferable content of silicon (Si) may be0.02% or more, and the more preferable content of silicon (Si) may be0.05% or more. However, when the content of silicon (Si) exceeds acertain level, a problem of plating defects, such as non-plating, may beinduced during plating, and weldability of a steel sheet may be lowered,so the present invention may limit the upper limit of the silicon (Si)content to 4.0%. The preferable upper limit of the content of silicon(Si) may be 3.8%, and the more preferable upper limit of the content ofsilicon (Si) may be 3.5%.

Aluminum (Al): 5.0% or less (excluding 0%)

Aluminum (Al) is an element that performs deoxidation by combining withoxygen in steel. In addition, aluminum (Al) is also an element forstabilizing the retained austenite by suppressing precipitation ofcementite like silicon (Si). Therefore, in the present invention,aluminum (Al) may be necessarily added to achieve such an effect. Apreferable content of aluminum (Al) may be 0.05% or more, and a morepreferable content of aluminum (Al) may be 0.1% or more. On the otherhand, when aluminum (Al) is excessively added, inclusions in a steelsheet increase, and the workability of the steel sheet may be lowered,so the present invention may limit the upper limit of the content ofaluminum (Al) to 5.0%. The preferable upper limit of the content ofaluminum (Al) may be 4.75%, and the more preferable upper limit of thecontent of aluminum (Al) may be 4.5%.

Meanwhile, the total content (Si+Al) of silicon (Si) and aluminum (Al)is preferably 1.0 to 6.0%. Since silicon (Si) and aluminum (Al) arecomponents that affect microstructure formation in the presentinvention, and thus, affect ductility, bending formability, and holeexpansion ratio, the total content of silicon (Si) and aluminum (Al) ispreferably 1.0 to 6.0%. The more preferable total content (Si+Al) ofsilicon (Si) and aluminum (Al) may be 1.5% or more, and may be 4.0% orless.

Manganese (Mn): 0.9 to 5.0%

Manganese (Mn) is a useful element for increasing both strength andductility. Therefore, in the present disclosure, a lower limit of acontent of manganese (Mn) may be limited to 0.9% in order to achievesuch an effect. A preferable lower limit of the content of manganese(Mn) may be 1.0%, and a more preferable lower limit of the content ofmanganese (Mn) may be 1.1%. On the other hand, when manganese (Mn) isexcessively added, the bainite transformation time increases and aconcentration of carbon (C) in the austenite becomes insufficient, sothere is a problem in that the desired austenite fraction may not besecured. Therefore, an upper limit of the content of manganese (Mn) ofthe present disclosure may be limited to 5.0%. A preferable upper limitof the content of manganese (Mn) may be 4.7%, and a more preferableupper limit of the content of manganese (Mn) may be 4.5%.

Phosphorus (P): 0.15% or less (including 0%)

Phosphorus (P) is an element that is included as an impurity anddeteriorates impact toughness. Therefore, it is preferable to manage thecontent of phosphorus (P) to 0.15% or less.

Sulfur(S): 0.03% or less (including 0%)

Sulfur (S) is an element that is included as an impurity to form MnS ina steel sheet and deteriorate ductility. Therefore, the content ofsulfur (S) is preferably 0.03% or less.

Nitrogen (N): 0.03% or less (including 0%)

Nitrogen (N) is an element that is contained as an impurity and formsnitride during continuous casting to causes cracks of slab. Therefore,the content of nitrogen (N) is preferably 0.03% or less.

Meanwhile, the steel sheet of the present invention has an alloycomposition that may be additionally included in addition to theabove-described alloy components, which will be described in detailbelow.

One or more of titanium (Ti): 0 to 0.5%, niobium (Nb): 0 to 0.5%, andvanadium (V): 0 to 0.5%

Titanium (Ti), niobium (Nb), and vanadium (V) are elements that makeprecipitates and refine crystal grains, and are elements that alsocontribute to the improvement in strength and impact toughness of asteel sheet, and therefore, in the present invention, one or more oftitanium (Ti), niobium (Nb), and vanadium (V) may be added to achievesuch an effect. However, when the content of titanium (Ti), niobium(Nb), and vanadium (V) exceed a certain level, respectively, excessiveprecipitates are formed to lower impact toughness and increasemanufacturing cost, so the present invention may limit the content oftitanium (Ti), niobium (Nb), and vanadium (V) to 0.5% or less,respectively.

One or more of chromium (Cr): 0 to 3.0% and molybdenum (Mo): 0 to 3.0%

Since chromium (Cr) and molybdenum (Mo) are elements that not onlysuppress austenite decomposition during alloying treatment, but alsostabilize austenite like manganese (Mn), the present invention may addone or more of chromium (Cr) and molybdenum (Mo) to achieve such aneffect. However, when the content of chromium (Cr) and molybdenum (Mo)exceeds a certain level, the bainite transformation time increases andthe concentration of carbon (C) in austenite becomes insufficient, sothe desired retained austenite fraction may not be secured. Therefore,the present invention may limit the content of chromium (Cr) andmolybdenum (Mo) to 3.0% or less, respectively.

One or More of Cu: 0 to 4.5% and Ni: 0 to 4.5%

Copper (Cu) and nickel (Ni) are elements that stabilize austenite andsuppress corrosion. In addition, copper (Cu) and nickel (Ni) are alsoelements that are concentrated on a surface of a steel sheet to preventhydrogen from intruding into the steel sheet, to thereby suppresshydrogen delayed destruction. Accordingly, in the present invention, oneor more of copper (Cu) and nickel (Ni) may be added to achieve such aneffect. However, when the content of copper (Cu) and nickel (Ni) exceedsa certain level, not only excessive characteristic effects, but also anincrease in manufacturing cost is induced, so the present invention maylimit the content of copper (Cu) and nickel (Ni) to 4.5% or less,respectively.

Boron (B): 0 to 0.005%

Boron (B) is an element that improves hardenability to increasestrength, and is also an element that suppresses nucleation of grainboundaries. Therefore, in the present invention, boron (B) may be addedto achieve such an effect. However, when the content of boron (B)exceeds a certain level, not only excessive characteristic effects, butalso an increases in manufacturing cost is induced, so the presentinvention may limit the content of boron (B) to 0.005% or less.

One or more of calcium (Ca): 0 to 0.05%, Magnesium (Mg): 0 to 0.05%, andrare earth element (REM) excluding yttrium (Y): 0 to 0.05%

Here, the rare earth element (REM) is scandium (Sc), yttrium (Y), and alanthanide element. Since calcium (Ca), magnesium (Mg), and the rareearth element (REM) excluding yttrium (Y) are elements that contributeto the improvement in ductility of a steel sheet by spheroidizingsulfides, in the present invention, one or more of calcium (Ca),magnesium (Mg), and the rare earth element (REM) excluding yttrium (Y)may be added to achieve such an effect. However, when the content ofcalcium (Ca), magnesium (Mg), and the rare earth element (REM) excludingyttrium (Y) exceeds a certain level, not only excessive characteristiceffects, but also an increase in manufacturing cost are induced, so thepresent invention may limit the content of calcium (Ca), magnesium (Mg),and the rare earth element (REM) excluding yttrium (Y) to 0.05% or less,respectively.

One or more of tungsten (W): 0 to 0.5% and zirconium (Zr): 0 to 0.5%

Since tungsten (W) and zirconium (Zr) are elements that increasestrength of a steel sheet by improving hardenability, in the presentinvention, one or more of tungsten (W) and zirconium (Zr) may be addedto achieve such an effect. However, when the content of tungsten (W) andzirconium (Zr) exceeds a certain level, not only excessivecharacteristic effects, but also an increase in manufacturing cost areinduced, so the present invention may limit the content of tungsten (W)and zirconium (Zr) to 0.5% or less, respectively.

One or More of Antimony (Sb): 0 to 0.5% and Tin (Sn): 0 to 0.5%

Since antimony (Sb) and tin (Sn) are elements that improve platingwettability and plating adhesion of a steel sheet, in the presentinvention, one or more of antimony (Sb) and tin (Sn) may be added toachieve such an effect. However, when the content of antimony (Sb) andtin (Sn) exceeds a certain level, brittleness of a steel sheetincreases, and thus, cracks may occur during hot working or coldworking, so the present invention may limit the content of antimony (Sb)and tin (Sn) to 0.5% or less, respectively.

One or More of Yttrium (Y): 0 to 0.2% and Hafnium (Hf): 0 to 0.2%

Since yttrium (Y) and hafnium (Hf) are elements that improve corrosionresistance of a steel sheet, in the present invention, one or more ofthe yttrium (Y) and hafnium (Hf) may be added to achieve such an effect.However, when the content of yttrium (Y) and hafnium (Hf) exceeds acertain level, the ductility of the steel sheet may deteriorate, so thepresent invention may limit the content of yttrium (Y) and hafnium (Hf)to 0.2% or less, respectively.

Cobalt (Co): 0 to 1.5%

Since cobalt (Co) is an element that promotes bainite transformation toincrease a TRIP effect, in the present invention, cobalt (Co) may beadded to achieve such an effect. However, when the content of cobalt(Co) exceeds a certain level, since weldability and ductility of a steelsheet may deteriorate, the present invention may limit the content ofcobalt (Co) to 1.5% or less.

The high strength steel sheet having superior workability according toan aspect of the present disclosure may include a balance of Fe andother unavoidable impurities in addition to the components describedabove. However, in a general manufacturing process, unintendedimpurities may inevitably be mixed from a raw material or thesurrounding environment, and thus, these impurities may not becompletely excluded. Since these impurities are known to those skilledin the art, all the contents are not specifically mentioned in thepresent specification. In addition, additional addition of effectivecomponents other than the above-described components is not entirelyexcluded.

The high strength steel sheet having superior workability according toan aspect of the present invention may include, as microstructures,ferrite which is a soft structure, and tempered martensite, bainite, andretained austenite which are hard structures. Here, the soft structureand the hard structure may be interpreted as a concept distinguished bya relative hardness difference.

As a preferred example, the microstructure of the high strength steelsheet having superior workability according to an aspect of the presentinvention may include, by volume fraction, 30 to 70% of temperedmartensite, 10 to 45% of bainite, 10 to 40% of retained austenite, 3 to20% of ferrite, and unavoidable microstructure components. As theunavoidable structure of the present invention, fresh martensite,perlite, martensite austenite constituent (M-A), and the like may beincluded. When the fresh martensite or the pearlite is excessivelyformed, the workability of the steel sheet may be lowered or thefraction of the retained austenite may be lowered.

The high strength steel sheet having superior workability according toan aspect of the present invention, as shown in the following[Relational Expression 1], a ratio of an average nanohardness value([H]_(F), Hv) of the soft structure (ferrite) to an average nanohardnessvalue ([H]_(TM+B+γ), Hv) of the hard structure (tempered martensite,bainite, and retained austenite) may satisfy a range of 0.4 to 0.9.

0.4≤[H]_(F)/[H]_(TM+B+γ)≤0.9  [Relational Expression 1]

The nanohardness values of the hard and soft structures may be measuredusing a nanoindenter (FISCHERSCOPE HM2000). Specifically, afterelectropolishing the surface of the steel sheet, the hard and softstructures are randomly measured at 20 points or more under thecondition of an indentation load of 10,000 μN, and the averagenanohardness value of the hard and soft structures may be calculatedbased on the measured values.

In addition, in the high strength steel sheet having superiorworkability according to an aspect of the present invention, since abalance B_(T·E) of tensile strength and elongation expressed by thefollowing [Relational Expression 2] is 22,000 (MPa %) or more, a balanceB_(T·H) of tensile strength and hole expansion ratio expressed by thefollowing [Relational Expression 3] is 7*10⁶ (MPa²%^(1/2)) or more, andbendability B_(R) expressed by the following [Relational Expression 4]satisfies a range of 0.5 to 3.0, it may have an superior balance ofstrength and ductility, a balance of strength and a hole expansionratio, and superb bending formability.

B _(T·E)=[Tensile Strength(TS,MPa)]*[Elongation(El,%)]  [RelationalExpression 2]

B _(T·H)=[Tensile Strength(TS,MPa)]²*[Hole ExpansionRatio(HER,%)]^(1/2)  [Relational Expression 3]

B _(R) =R/t  [Relational Expression 4]

In the above Relational Expression 4, R is a minimum bending radius (mm)at which cracks do not occur after a 90° bending test, and t is athickness (mm) of the steel sheet.

In the present invention, it is important to stabilize retainedaustenite of a steel sheet because it is intended to simultaneouslysecure superb ductility and bending formability as well as high strengthproperties. In order to stabilize the retained austenite, it isnecessary to concentrate carbon (C) and manganese (Mn) in the ferrite,bainite, and tempered martensite of the steel sheet into austenite.However, when carbon (C) is concentrated into austenite by usingferrite, the strength of the steel sheet may be insufficient due to thelow strength characteristics of ferrite, and excessive inter-phasehardness difference may occur, thereby reducing the hole expansion ratio(HER). Therefore, it is intended to concentrate carbon (C) and manganese(Mn) into austenite by using the bainite and tempered martensite.

When the content of silicon (Si) and aluminum (Al) in the retainedaustenite is limited to a certain range, carbon (C) and manganese (Mn)may be concentrated in large amounts from bainite and temperedmartensite into retained austenite, thereby effectively stabilizing theretained austenite. In addition, by limiting the content of silicon (Si)and aluminum (Al) in austenite to a certain range, it is possible toincrease the content of silicon (Si) and aluminum (Al) in ferrite. Asthe content of silicon (Si) and aluminum (Al) in the ferrite increases,the hardness of the ferrite increases, so it is possible to effectivelyreduce an inter-phase hardness difference between ferrite which is asoft structure and tempered martensite, bainite, and retained austenitewhich are a hard structure.

When the ratio of the average nanohardness value ([H]_(F), Hv) of thesoft structure (ferrite) to the average nanohardness value([H]_(TM+B+γ), Hv) of the hard structure (tempered martensite, bainite,and retained austenite) is greater than a certain level, the inter-phasehardness difference between the soft structure (ferrite) and the hardstructure (tempered martensite, bainite, and retained austenite) islowered, so it is possible to secure a desired balance (TS×E1) oftensile strength and elongation, a balance (TS²×HER^(1/2)) of tensilestrength and hole expansion ratio, and bendability (R/t). On the otherhand, when the ratio of the average nanohardness value ([H]_(F), Hv) ofthe soft structure (ferrite) to the average nanohardness value([H]_(TM+B+γ), Hv) of the hard structure (tempered martensite, bainite,and retained austenite) is excessive, the ferrite is excessivelyhardened and the workability is rather lowered, so the desired balance(TS×E1) of tensile strength and elongation, the balance of tensilestrength and hole expansion ratio (TS²×HER^(1/2)), and the bendability(R/t) may not all be secured Therefore, the present invention may limitthe ratio of the average nanohardness value ([H]_(F), Hv) of the softstructure to the average nanohardness value ([H]_(TM+B+γ), Hv) of thehard structure (tempered martensite, bainite, and retained austenite) toa range of 0.4 to 0.9.

A steel sheet including retained austenite has superb ductility andbending formability due to transformation-induced plasticity that occursduring transformation from austenite to martensite during processing.When the fraction of the retained austenite is less than a certainlevel, the balance (TS×E1) of tensile strength and elongation may beless than 22,000 MPa %, or the bendability (R/t) may exceed 3.0.Meanwhile, when the fraction of the retained austenite exceeds a certainlevel, local elongation may be lowered. Accordingly, in the presentinvention, the fraction of the retained austenite may be limited to arange of 10 to 40 vol % in order to obtain a steel sheet having abalance (TS×E1) of tensile strength and elongation and superbbendability (R/t).

Meanwhile, both untempered martensite (fresh martensite) and temperedmartensite are microstructures that improve the strength of the steelsheet. However, compared with the tempered martensite, fresh martensitehas a characteristic of greatly reducing the ductility and the holeexpansion ratio of the steel sheet. This is because the microstructureof the tempered martensite is softened by the tempering heat treatment.Therefore, in the present invention, it is preferable to use temperedmartensite to provide a steel sheet having a balance of strength andductility, a balance of strength and hole expansion ratio, and superbworkability. When the fraction of the tempered martensite is less than acertain level, it is difficult to secure the balance (TS×E1) of tensilestrength and elongation of 22,000 MPa % or more or the balance(TS²×HER^(1/2)) of tensile strength and hole expansion ratio of 7*10⁶(MPa²%^(1/2)) or more, and when the fraction of the tempered martensiteexceeds a certain level, ductility and workability is lowered, and thebalance (TS×E1) of tensile strength and elongation is less than 22,000MPa %, or bendability (R/t) exceeds 3.0, which is not preferable.Therefore, in the present invention, the fraction of the temperedmartensite may be limited to 30 to 70 vol % to obtain a steel sheethaving the balance (TS×El) of tensile strength and elongation, thebalance (TS²×HER^(1/2)) of tensile strength and hole expansion ratio,and superb bendability (R/t).

In order to improve the balance (TS×El) of tensile strength andelongation, the balance (TS²×HER^(1/2)) of tensile strength and holeexpansion ratio, and the bendability (R/t), it is preferable thatbainite is appropriately included as the microstructure. As long as afraction of bainite is a certain level or more, it is possible to securethe balance (TS×El) of tensile strength and elongation of 22,000 MPa %or more, the balance (TS²×HER^(1/2)) of tensile strength and holeexpansion ratio of 7*10⁶ (MPa²%^(1/2)) or more and the bendability (R/t)of 0.5 to 3.0. On the other hand, when the fraction of bainite isexcessive, the decrease in the fraction of tempered martensite isnecessarily accompanied, so the present invention may not secure thedesired balance (TS×El) of tensile strength and elongation, the balance(TS²×HER^(1/2)) of tensile strength and hole expansion ratio, andbendability (R/t). Accordingly, the present invention may limit thefraction of bainite to a range of 10 to 45 vol %.

Since ferrite is an element contributing to improvement in ductility,the present invention may secure the desired balance (TS×El) of tensilestrength and elongation, as long as the fraction of ferrite is a certainlevel or more. However, when the fraction of ferrite is excessive, theinter-phase hardness difference increases and the hole expansion ratio(HER) may decrease, so the present invention may not secure the desiredbalance (TS²×HER^(1/2)) of tensile strength and hole expansion ratio.Accordingly, the present invention may limit the fraction of ferrite toa range of 3 to 20 vol %.

Hereinafter, an example of a method for manufacturing a steel sheet ofthe present invention will be described in detail.

A manufacturing method for high-strength steel sheet according to anaspect of the present invention may include: preparing a steel slabhaving a predetermined component; heating and hot rolling the steelslab; coiling the hot-rolled steel sheet; performing a hot-rolledannealing heat treatment on the coiled steel sheet in a temperaturewithin a range of 650 to 850° C. for 600 to 1700 seconds; cold rollingthe hot-rolled annealing heat-treated steel sheet; heating (primaryheating) the cold-rolled steel sheet to a temperature within a range ofAc1 or higher and less than Ac3, and maintaining (primary maintaining)the cold-rolled steel sheet for 50 seconds or more; cooling (primarycooling) the primarily heated steel sheet to a temperature within arange of 100 to 300° C. at an average cooling rate of 1° C./s or more;heating (secondary heating) the primarily cooled steel sheet to atemperature within a range of 300 to 500° C. at an average temperatureincrease rate of 5° C./s or more, and maintaining (secondarilymaintaining) the primarily cooled steel sheet for 50 seconds or more;and cooling (secondarily cooling) the secondarily heated steel sheet toroom temperature.

Preparation and Heating of Steel Slab

A steel slab having a predetermined component is prepared. Since thesteel slab according to the present invention includes an alloycomposition corresponding to an alloy composition of the steel sheetdescribed above, the description of the alloy compositions of the slabis replaced by the description of the alloy composition of the steelsheet described above.

The prepared steel slab may be heated to a temperature within a certaintemperature range, and the heating temperature of the steel slab at thistime may be in the range of 1000 to 1350° C. This is because, when theheating temperature of the steel slab is less than 1000° C., the steelslab may be hot rolled in the temperature range below the desired finishhot rolling temperature range, and when the heating temperature of thesteel slab exceeds 1350° C., the temperature reaches a melting point ofsteel, and thus, the steel slab is melted.

Hot Rolling and Coiling

The heated steel slab may be hot rolled, and thus, provided as ahot-rolled steel sheet. During the hot rolling, the finish hot rollingtemperature is preferably in the range of 800 to 1000° C. When thefinish hot rolling temperature is less than 800° C., an excessiverolling load may be a problem, and when the finish hot rollingtemperature exceeds 1000° C., grains of the hot-rolled steel sheet arecoarsely formed, which may cause a deterioration in physical propertiesof the final steel sheet.

The hot-rolled steel sheet after the hot rolling has been completed maybe cooled at an average cooling rate of 10° C./s or more, and may becoiled at a temperature of 300 to 600° C. When the coiling temperatureis less than 300° C., the coiling is not easy, and when the coilingtemperature exceeds 600° C., a surface scale is formed to the inside ofthe hot-rolled steel sheet, which may make pickling difficult.

Hot-Rolled Annealing Heat Treatment

It is preferable to perform a hot rolling annealing heat treatmentprocess in order to facilitate pickling and cold rolling, which aresubsequent processes after the coiling. The hot rolling annealing heattreatment may be performed in a temperature within a range of 650 to850° C. for 600 to 1700 seconds. When the hot rolling annealing heattreatment temperature is less than 650° C. or the hot rolling annealingheat treatment time is less than 600 seconds, the strength of the hotrolling annealing heat-treated steel sheet increases, and thus,subsequent cold rolling may not be easy. On the other hand, when the hotrolling annealing heat treatment temperature exceeds 850° C. or the hotrolling annealing heat treatment time exceeds 1700 seconds, the picklingmay not be easy due to a scale formed deep inside the steel sheet.

Pickling and Cold Rolling

After the hot rolling annealing heat treatment, in order to remove thescale generated on the surface of the steel sheet, the pickling may beperformed, and the cold rolling may be performed. Although theconditions of the pickling and cold rolling are not particularly limitedin the present invention, the cold rolling is preferably performed at acumulative reduction ratio of 30 to 90%. When the cumulative reductionratio of the cold rolling exceeds 90%, it may be difficult to performthe cold rolling in a short time due to the high strength of the steelsheet.

The cold-rolled steel sheet may be manufactured as a non-platedcold-rolled steel sheet through the annealing heat treatment process, ormay be manufactured as a plated steel sheet through a plating process toimpart corrosion resistance. As the plating, plating methods such ashot-dip galvanizing, electro-galvanizing, and hot-dip aluminum platingmay be applied, and the method and type are not particularly limited.

Annealing Heat Treatment

In the present invention, in order to simultaneously secure the strengthand workability of the steel sheet, the annealing heat treatment processis performed.

The cold-rolled steel sheet is heated (primarily heated) to atemperature within a range of Ac1 or higher and less than Ac3 (two-phaseregion), and held (primarily held) in the temperature range for 50seconds or more. The primary heating or primary maintaining temperatureis Ac3 or higher (single-phase region), the desired ferrite structuremay not be realized, so the desired level of [H]_(F)/[H]_(TM+B+γ), andthe balance (TS²×HER^(1/2)) of tensile strength and hole expansion ratiomay be implemented. In addition, when the primary heating or primarymaintaining temperature is in a temperature range less than Ac1, thereis a fear that sufficient heating is not made, and thus, themicrostructure desired by the present invention may not be implementedeven by subsequent heat treatment. The average temperature increase rateof the primary heating may be 5° C./s or more.

When the primary maintaining time is less than 50 seconds, the structuremay not be sufficiently homogenized and the physical properties of thesteel sheet may be lowered. The upper limit of the primary maintainingtime is not particularly limited, but the primary heating time ispreferably limited to 1200 seconds or less in order to prevent thedecrease in toughness due to the coarsening of grains.

After the primary maintaining, the primarily heated steel sheet may becooled (primarily cooled) to a primary cooling stop temperature of 100to 300° C. at a primary cooling rate of an average cooling rate of 1°C./s or more. The upper limit of the primary cooling rate does not needto be particularly specified, but is preferably limited to 100° C./s orless. When the primary cooling stop temperature exceeds 100° C., thetempered martensite is excessively formed and the amount of retainedaustenite formed is insufficient, so [H]_(F)/[H]_(TM+B+γ), the balance(TS×E1) of tensile strength and elongation, and the bendability (R/t)may be lowered. On the other hand, when the primary cooling stoptemperature exceeds 300° C., the bainite is excessively formed and theamount of tempered martensite formed is insufficient, so the balance(TS×El) of tensile strength and elongation of the steel sheet, and thebalance (TS²×HER^(1/2)) of tensile strength and hole expansion ratio ofthe steel sheet may be lowered.

After the primary cooling, the primarily cooled steel sheet may beheated (secondarily heated) to a secondary heating temperature of 300 to500° C. at a secondary heating rate of an average temperature increaserate of 5° C./s or more, and may be maintained (secondarily maintained)for 50 seconds or more in the temperature range. The upper limit of thesecondary temperature increase rate does not need to be particularlyspecified, but is preferably limited to 100° C./s or less. When thesecondary heating or secondary maintaining temperature is less than 300°C., or the maintaining time is less than 50 seconds, the temperedmartensite is excessively formed and the control of Si and Al content inthe retained austenite is insufficient, so the desired fraction of theretained austenite is difficult to obtain. As a result,[H]_(F)/[H]_(TM+B+γ), the balance (TS×El) of tensile strength andelongation and the bending workability (R/t) may be lowered. On theother hand, when the secondary heating or maintaining temperatureexceeds 500° C. or the secondary maintaining time is 172,000 seconds ormore, it is difficult to secure the fraction of the retained austenitebecause the control of Si and Al content in the retained austenite isinsufficient. As a result, [H]_(F)/[H]_(TM+B+γ), and the balance (TS×El)of tensile strength and elongation may be lowered.

After the secondary maintaining, it is preferable to cool (secondarilycool) the secondarily heated steel sheet to room temperature at anaverage cooling rate of 1° C./s or more.

The high strength steel sheet having superior workability manufacturedby the above-described manufacturing method may include, as amicrostructure, tempered martensite, bainite, retained austenite, andferrite, and as a preferred example, may include, by the volumefraction, 30 to 70% of tempered martensite, 10 to 45% of bainite, 10 to40% of retained austenite, 3 to 20% of ferrite, and unavoidablestructures.

In addition, in the high-strength steel sheet having superiorworkability manufactured by the above-described manufacturing method, asin the following [Relational Expression 1], the ratio of the averagenanohardness value ([H]_(F), Hv) of the soft structure (ferrite) to theaverage nanohardness value ([H]_(TM+B+γ), Hv) of the hard structures(tempered martensite, bainite, and retained austenite) may satisfy therange of 0.4 to 0.9, the balance B_(T·E) of tensile strength andelongation expressed by the following [Relational Expression 2] is22,000 (MPa %) or more, and the balance B_(T·H) of tensile strength andelongation expressed by the following [Relational Expression 3] may be7*10⁶ (MPa²%^(1/2)) or more, and the bendability B_(A) expressed by thefollowing [Relational Expression 4] may satisfy the range of 0.5 to 3.0.

0.4≤[H]_(F)/[H]_(TM+B+γ)≤0.9  [Relational Expression 1]

B _(T·E)=[Tensile Strength(TS,MPa)]*[Elongation(EL,%)]  [RelationalExpression 2]

B _(T·H)=[Tensile Strength(TS,MPa)]²*[Hole ExpansionRatio(HER,%)]^(1/2)  [Relational Expression 3]

B _(R) =R/t  [Relational Expression 4]

In the above Relational Expression 4, R is a minimum bending radius (mm)at which cracks do not occur after a 90° bending test, and t is athickness (mm) of the steel sheet.

MODE FOR INVENTION

Hereinafter, a high strength steel sheet having superior workability anda method for manufacturing same according to an aspect of the presentinvention will be described in more detail. It should be noted that thefollowing examples are only for the understanding of the presentinvention, and are not intended to specify the scope of the presentinvention. The scope of the present invention is determined by mattersdescribed in claims and matters reasonably inferred therefrom.

Inventive Example

A steel slab having a thickness of 100 mm having alloy compositions (abalance of Fe and unavoidable impurities) shown in Table 1 below wasprepared, heated at 1200° C., and then was subjected to finish hotrolling at 900° C. Thereafter, the steel slab was cooled at an averagecooling rate of 30° C./s, and coiled at a coiling temperature of Tables2 and 3 to manufacture a hot-rolled steel sheet having a thickness of 3mm. The hot-rolled steel sheet was subjected to hot rolling annealingheat treatment under the conditions of Tables 2 and 3. Thereafter, afterremoving a surface scale by pickling, cold rolling was performed to athickness of 1.5 mm.

Thereafter, the heat treatment was performed under the annealing heattreatment conditions disclosed in Tables 2 to 5 to manufacture the steelsheet.

The microstructure of the thus prepared steel sheet was observed, andthe results were shown in Tables 6 and 7. Among the microstructures,ferrite (F), bainite (B) tempered martensite (TM), and pearlite (P) wereobserved through SEM after nital-etching a polished specimen crosssection. The fractions of bainite and tempered martensite, which aredifficult to distinguish among them, were calculated using an expansioncurve after evaluation of dilatation. Meanwhile, since fresh martensite(FM) and retained austenite (retained γ) are also difficult todistinguish, a value obtained by subtracting the fraction of retainedaustenite calculated by X-ray diffraction method from the fraction ofmartensite and retained austenite observed by the SEM was determined asthe fraction of the fresh martensite.

Meanwhile, [H]_(F)/[H]_(TM+B+γ), a balance (TS×E1) of tensile strengthand elongation, a balance (TS²×HER^(1/2)) of tensile strength and holeexpansion ratio, and bendability (R/t) were observed, and the resultswere shown in Tables 8 and 9.

Tensile strength (TS) and elongation (El) were evaluated through atensile test, and the tensile strength (TS) and the elongation (El) weremeasured by evaluating the specimens collected in accordance with JISNo. 5 standard based on a 90° direction with respect to a rollingdirection of a rolled sheet. The bendability (R/t) was evaluated by aV-bending test, and calculated by collecting a specimen based on the 90°direction with respect to the rolling direction of the rolled sheet andwas determined and calculated as a value obtained by dividing a minimumbending radius R (mm), at which cracks do not occur after a 90° bendingtest, by a thickness t (mm) of a sheet. The hole expansion ratio (HER)was evaluated through the hole expansion test, and was calculated by thefollowing [Relational Expression 5] by, after forming a punching hole(die inner diameter of 10.3 mm, clearance of 12.5%) of 10 mmØ, insertinga conical punch having an apex angle of 60° into a punching hole in adirection in which a burr of a punching hole faces outward, and thencompressing and expanding a peripheral portion of the punching hole at amoving speed of 20 mm/min.

Hole Expansion Ratio(HER,%)={(D−D ₀)/D ₀}×100  [Relational Expression 5]

In the above Relational Expression 5, D is a hole diameter (mm) whencracks penetrate through the steel plate along the thickness direction,and D₀ is the initial hole diameter (mm).

Nanohardness values of hard and soft structures were measured using thenanoindentation method. Specifically, after electropolishing surfaces ofeach specimen, the hard and soft structures were randomly measured at 20points or more under the condition of an indentation load of 10,000 ρNusing a nanoindenter (FISCHERSCOPE HM2000), and the average nanohardnessvalue of the hard and soft structures was calculated based on themeasured values.

TABLE 1 Steel Chemical Component (wt %) Type C Si Mn P S Al N Cr MoOthers A 0.33 1.77 2.13 0.008 0.0011 0.45 0.0025 0.53 B 0.34 2.52 2.260.013 0.0009 0.51 0.0033 0.31 0.24 C 0.33 2.28 2.15 0.009 0.0011 0.440.0024 0.53 D 0.35 2.47 3.58 0.012 0.0012 0.63 0.0023 0.49 E 0.41 1.592.45 0.009 0.0008 0.49 0.0034 F 0.55 1.67 2.33 0.008 0.0010 0.88 0.0031G 0.67 1.72 1.14 0.010 0.0011 0.95 0.0026 H 0.34 1.88 2.26 0.011 0.00091.19 0.0034 I 0.37 1.23 1.68 0.009 0.0010 2.47 0.0027 J 0.33 0.04 2.710.008 0.0011 4.46 0.0031 Ti: 0.05 K 0.41 2.26 2.54 0.011 0.0011 0.490.0025 Nb: 0.04 L 0.43 2.41 2.37 0.013 0.0013 0.33 0.0022 V: 0.06 M 0.351.37 1.90 0.009 0.0008 0.50 0.0036 Ni: 0.31 N 0.33 1.45 2.29 0.0100.0011 0.61 0.0027 Cu: 0.35 O 0.36 1.32 2.63 0.013 0.0007 0.59 0.0031 B:0.0024 P 0.30 1.49 2.91 0.009 0.0009 0.52 0.0020 Ca: 0.002 Q 0.34 1.922.58 0.011 0.0010 0.47 0.0022 REM: 0.001 R 0.41 1.37 2.35 0.008 0.00110.54 0.0036 Mg: 0.002 S 0.42 1.46 2.07 0.010 0.0010 0.45 0.0029 W: 0.14T 0.39 1.65 2.23 0.011 0.0012 0.63 0.0030 Zr: 0.12 U 0.37 1.41 2.610.009 0.0013 0.56 0.0034 Sb: 0.02 V 0.34 1.55 2.88 0.008 0.0010 0.490.0027 Sn: 0.03 W 0.34 1.32 2.42 0.012 0.0009 0.52 0.0023 Y: 0.01 X 0.273.68 1.90 0.011 0.0010 0.47 0.0032 Hf: 0.02 Y 0.36 2.23 2.43 0.0090.0011 0.44 0.0035 Co: 0.31 XA 0.23 1.45 2.58 0.011 0.0009 0.51 0.0028XB 0.78 1.59 2.27 0.008 0.0010 0.39 0.0024 XC 0.33 0.02 2.42 0.0090.0013 0.03 0.0025 XD 0.35 4.17 2.83 0.011 0.0009 0.04 0.0037 XE 0.410.03 2.35 0.009 0.0010 5.27 0.0026 XF 0.42 1.48 0.76 0.010 0.0008 0.450.0031 XG 0.37 1.59 5.25 0.0011 0.0010 0.50 0.0026 XH 0.36 2.35 2.470.009 0.0009 0.46 0.0033 3.27 XI 0.34 2.26 2.26 0.010 0.0008 0.53 0.00293.28

TABLE 2 Coiling Annealing Annealing Primary Primary temperaturetemperature time of average maintaining Primary of hot-rolled ofhot-rolled hot-rolled heating temperature maintaining Specimen Steelsteel sheet steel sheet steel sheet rate section time No. type (° C.) (°C.) (s) (° C./s) (° C.) (s) 1 A 550 750 1300 10 Two-phase region 120 2 A500 900 1100 Poor pickling 3 A 550 600 1200 Occurrence of fractureduring cold rolling 4 A 450 700 1800 Poor pickling 5 A 550 750 500Occurrence of fracture during cold rolling 6 A 500 700 1100 10Single-phase region 120 7 A 500 650 1300 10 Two-phase region 120 8 B 550800 1000 10 Two-phase region 120 9 B 500 700 1200 10 Two-phase region120 10 B 450 750 1300 10 Two-phase region 120 11 C 500 650 1000 10Two-phase region 120 12 C 550 700 1200 10 Two-phase region 120 13 C 500700 1100 10 Two-phase region 120 14 C 500 750 1300 10 Two-phase region120 15 C 450 800 600 10 Two-phase region 120 16 C 500 750 1200 10Two-phase region 120 17 C 550 700 1700 10 Two-phase region 120 18 D 500750 1200 10 Two-phase region 120 19 E 500 700 900 10 Two-phase region120 20 F 500 850 1200 10 Two-phase region 120 21 G 350 700 1500 10Two-phase region 120 22 H 450 750 1300 10 Two-phase region 120 23 I 500700 1400 10 Two-phase region 120 24 J 450 700 1000 10 Two-phase region120 25 K 550 750 900 10 Two-phase region 120

TABLE 3 Coiling Annealing Annealing Primary Primary temperaturetemperature time of average maintaining Primary of hot-rolled ofhot-rolled hot-rolled heating temperature maintaining Specimen Steelsteel sheet steel sheet steel sheet rate section time No. type (° C.) (°C.) (s) (° C./s) (° C.) (s) 26 L 550 700 1300 10 Two-phase region 120 27M 500 750 1100 10 Two-phase region 120 28 N 550 750 1200 10 Two-phaseregion 120 29 O 450 700 900 10 Two-phase region 120 30 P 550 700 1500 10Two-phase region 120 31 Q 500 700 1300 10 Two-phase region 120 32 R 450800 900 10 Two-phase region 120 33 S 500 750 1000 10 Two-phase region120 34 T 550 700 1500 10 Two-phase region 120 35 U 500 800 1300 10Two-phase region 120 36 V 500 700 1400 10 Two-phase region 120 37 W 550750 1200 10 Two-phase region 120 38 X 450 700 1300 10 Two-phase region120 39 Y 550 750 1400 10 Two-phase region 120 40 XA 500 800 1200 10Two-phase region 120 41 XB 550 750 1300 10 Two-phase region 120 42 XC550 700 1200 10 Two-phase region 120 43 XD 500 750 900 10 Two-phaseregion 120 44 XE 500 750 1200 10 Two-phase region 120 45 XF 500 700 100010 Two-phase region 120 46 XG 450 700 1300 10 Two-phase region 120 47 XH500 800 1600 10 Two-phase region 120 48 XI 550 700 1300 10 Two-phaseregion 120

TABLE 4 Primary Primary Secondary Secondary average cooling averageSecondary Secondary average cooling stop heating maintaining maintainingcooling Specimen Steel rate temperature rate temperature time rate No.type (° C./s) (° C.) (° C./s) (° C.) (s) (° C./s) 1 A 20 220 15 400 30010 2 A Poor pickling 3 A Occurrence of fracture during cold rolling 4 APoor pickling 5 A Occurrence of fracture during cold rolling 6 A 20 22015 400 300 10 7 A 0.5 200 15 400 300 10 8 B 20 200 15 400 300 10 9 B 20180 15 400 300 10 10 B 20 240 15 350 600 10 11 C 20 140 15 400 300 10 12C 20 80 15 450 300 10 13 C 20 330 15 400 300 10 14 C 20 200 15 270 30010 15 C 20 180 15 530 300 10 16 C 20 240 15 400 40 10 17 C 20 200 15 350172,000 10 18 D 20 220 15 400 300 10 19 E 20 180 15 400 300 10 20 F 20220 15 450 300 10 21 G 20 280 15 300 300 10 22 H 20 200 15 450 300 10 23I 20 240 15 500 500 10 24 J 20 120 15 350 300 10 25 K 20 240 15 400 30010

TABLE 5 Primary Primary Secondary Secondary average cooling averageSecondary Secondary average cooling stop heating maintaining maintainingcooling Specimen Steel rate temperature rate temperature time rate No.type (° C./s) (° C.) (° C./s) (° C.) (s) (° C./s) 26 L 20 240 15 400 30010 27 M 20 200 15 400 300 10 28 N 20 200 15 450 300 10 29 O 20 240 15350 600 10 30 P 20 180 15 400 300 10 31 Q 20 220 15 400 300 10 32 R 20200 15 400 300 10 33 S 20 200 15 450 300 10 34 T 20 240 15 450 300 10 35U 20 220 15 400 300 10 36 V 20 200 15 450 300 10 37 W 20 200 15 400 30010 38 X 20 240 15 450 600 10 39 Y 20 220 15 400 300 10 40 XA 20 180 15300 300 10 41 XB 20 200 15 400 300 10 42 XC 20 180 15 400 300 10 43 XD20 200 15 450 600 10 44 XE 20 220 15 400 300 10 45 XF 20 200 15 450 30010 46 XG 20 200 15 400 300 10 47 XH 20 220 15 400 300 10 48 XI 20 200 15400 300 10

TABLE 6 Tempered Fresh Retained Specimen Steel Ferrite Bainitemartensite martensite austenite Perlite No. type (vol. %) (vol. %) (vol.%) (vol. %) (vol. %) (vol. %) 1 A 11 15 55 0 19 0 2 A Poor pickling 3 AOccurrence of fracture during cold rolling 4 A Poor pickling 5 AOccurrence of fracture during cold rolling 6 A 1 21 61 0 17 0 7 A 22 1348 0 5 12 8 B 5 18 61 0 16 0 9 B 12 15 54 1 18 0 10 B 8 16 56 0 20 0 11C 10 22 47 0 21 0 12 C 11 6 78 0 5 0 13 C 9 64 7 1 19 0 14 C 10 13 72 05 0 15 C 13 18 62 1 6 0 16 C 9 13 74 0 4 0 17 C 11 19 63 2 5 0 18 D 1017 54 0 19 0 19 E 13 14 58 0 15 0 20 F 7 22 52 1 18 0 21 G 6 43 36 0 150 22 H 18 16 49 0 17 0 23 I 11 18 57 0 14 0 24 J 9 16 56 1 18 0 25 K 1019 55 0 16 0

TABLE 7 Tempered Fresh Retained Specimen Steel Ferrite Bainitemartensite martensite austenite Perlite No. type (vol. %) (vol. %) (vol.%) (vol. %) (vol. %) (vol. %) 26 L 8 16 57 1 18 0 27 M 10 15 58 1 16 028 N 5 17 63 0 15 0 29 O 7 22 57 0 14 0 30 P 9 21 50 1 19 0 31 Q 11 1757 0 15 0 32 R 7 14 45 0 34 0 33 S 10 18 50 1 21 0 34 T 9 19 53 0 19 035 U 12 15 56 0 17 0 36 V 8 17 59 0 16 0 37 W 11 20 55 0 14 0 38 X 8 2154 0 17 0 39 Y 9 18 58 0 15 0 40 XA 10 16 61 0 13 0 41 XB 7 14 18 19 420 42 XC 8 21 66 2 3 0 43 XD 6 11 43 22 18 0 44 XE 5 16 44 20 15 0 45 XF7 18 62 0 5 8 46 XG 6 14 50 14 16 0 47 XH 8 17 43 19 13 0 48 XI 7 13 4916 15 0

TABLE 8 Specimen Steel [H]_(F)/ B_(T·E) B_(T·H) B_(R) No. type[H]_(TM+B+γ) (MPa %) (MPa² %^(1/2)) [R/t] 1 A 0.67 30,636 10,606,0401.95 2 A Poor pickling 3 A Occurrence of fracture during cold rolling 4A Poor pickling 5 A Occurrence of fracture during cold rolling 6 A 0.2128,891 6,393,193 2.42 7 A 0.94 18,879 8,475,048 2.54 8 B 0.72 30,51810,517,710 1.67 9 B 0.77 28,718 11,559,177 1.33 10 B 0.60 31,0859,926,629 1.58 11 C 0.66 30,639 12,510,645 1.87 12 C 0.95 17,2788,765,217 5.60 13 C 0.76 20,377 5,355,715 2.62 14 C 0.92 15,4817,284,461 5.36 15 C 0.95 18,648 9,179,726 2.04 16 C 0.93 16,19311,431,072 5.93 17 C 0.96 19,615 8,696,192 2.24 18 D 0.79 28,99610,252,535 1.29 19 E 0.69 30,498 8,626,877 0.67 20 F 0.71 31,8119,272,433 2.89 21 G 0.58 29,064 11,749,404 1.28 22 H 0.81 32,3088,878,145 1.92 23 I 0.43 29,967 10,849,320 2.34 24 J 0.87 30,8829,012,837 2.07 25 K 0.78 28,332 8,648,407 1.88

TABLE 9 Specimen Steel [H]_(F)/ B_(T·E) B_(T·H) B_(R) No. type[H]_(TM+B+γ) (MPa %) (MPa² %^(1/2)) [R/t] 26 L 0.75 30,157 8,058,9631.70 27 M 0.80 28,294 11,607,916 1.85 28 N 0.62 31,608 9,754,084 1.48 29O 0.67 33,459 10,481,642 2.16 30 P 0.72 30,238 12,929,614 2.33 31 Q 0.7032,795 9,497,041 2.59 32 R 0.77 29,766 8,331,161 2.48 33 S 0.69 31,54710,596,615 1.77 34 T 0.71 30,303 11,373,489 2.15 35 U 0.64 32,05610,905,303 2.19 36 V 0.45 31,529 9,571,137 1.99 37 W 0.59 29,8189,439,702 1.52 38 X 0.67 27,874 11,311,612 1.71 39 Y 0.54 30,3978,337,602 1.24 40 XA 0.51 19,508 6,015,204 2.08 41 XB 0.83 21,0755,643,588 6.56 42 XC 0.97 14,997 9,026,852 4.94 43 XD 0.78 26,9637,986,381 4.57 44 XE 0.66 27,856 10,871,201 6.49 45 XF 0.94 15,4208,540,137 2.21 46 XG 0.80 24,351 9,258,527 4.86 47 XH 0.74 26,75910,845,197 6.55 48 XI 0.71 27,882 11,049,872 4.73

As shown in Tables 1 to 9 above, it could be seen that the specimenssatisfying the conditions presented in the present inventionsimultaneously provide superior strength and workability since the valueof [H]_(F)/[H]_(TM+B+γ) satisfies the range of 0.4 to 0.9, the balance(TS×El) of tensile strength and elongation is 22,000 MPa % or more, thebalance (TS²×HER^(1/2)) of tensile strength and hole expansion ratio is7*10⁶ (MPa²%^(1/2)) or more, and the bendability (R/t) satisfies therange of 0.5 to 3.0.

It could be seen that, in specimens 2 to 5, since the alloy compositionrange of the present invention overlaps, but the hot rolling annealingtemperature and time are outside the range of the present invention, thepickling failure occurs or the fracture occurs during the cold rolling.

In specimen 6, the amount of ferrite formed was insufficient because theprimary heating or maintaining temperature in the annealing heattreatment process after the cold rolling exceeded the range limited bythe present invention. As a result, it could be seen that, in specimen6, [H]_(F)/[H]_(TM+B+γ) is less than 0.4, and the balance of tensilestrength and hole expansion ratio (TS²×HER^(1/2)) is less than 7*10⁶(MPa²%^(1/2)).

In specimen 7, the primary cooling rate in the annealing heat treatmentafter the cold rolling did not reach the range limited by the presentinvention, so the ferrite was excessively formed and the retainedaustenite was formed less. As a result, it could be seen that, inspecimen 7, [H]_(F)/[H]_(TM+B+γ) exceeds 0.9, and the balance (TS×E1) oftensile strength and elongation is less than 22,000 MPa %.

In specimen 12, the primary cooling stop temperature was low, so thattempered martensite was excessively formed and less retained austenitewas formed. As a result, it could be seen that, in specimen 12,[H]_(F)/[H]_(TM+B+γ) exceeds 0.9, the balance (TS×E1) of tensilestrength and elongation is less than 22,000 MPa %, and the bendability(R/t) exceeds 3.0.

In Specimen 13, the primary cooling stop temperature was high, so thatbainite was excessively formed and less tempered martensite was formed.As a result, it could be seen that, in specimen 13, the balance (TS×E1)of tensile strength and elongation is less than 22,000 MPa % and thebalance (TS²×HER^(1/2)) of tensile strength and hole expansion ratio isless than 7*10⁶ (MPa²%^(1/2))

In specimen 14, the secondary heating or maintaining temperature waslow, so that tempered martensite was excessively formed and lessretained austenite was formed. As a result, it could be seen that, inspecimen 14, [H]_(F)/[H]_(TM+B+γ) exceeds 0.9, the balance (TS×E1) oftensile strength and elongation is less than 22,000 MPa %, and thebendability (R/t) exceeds 3.0.

It could be seen that, in specimen 15, the secondary heating ormaintaining temperature is high, so the amount of retained austeniteformed is insufficient, [H]F/[H]_(TM+B+γ) exceeds 0.9, and the balance(TS×E1) of tensile strength and elongation is less than 22,000 MPa %.

In specimen 16, the secondary maintaining time was insufficient, so thattempered martensite was excessively formed and less retained austenitewas formed. As a result, it could be seen that, in specimen 16,[H]_(F)/[H]_(TM+B+γ) exceeds 0.9, the balance (TS×E1) of tensilestrength and elongation is less than 22,000 MPa %, and the bendability(R/t) exceeds 3.0.

It could be seen that, in specimen 17, the secondary maintaining timewas excessive, so the amount of retained austenite formed wasinsufficient, [H]_(F)/[H]_(TM+B+γ) exceeded 0.9, and the balance (TS×E1)of tensile strength and elongation was less than 22,000 MPa %.

Specimens 40 to 48 may satisfy the manufacturing conditions presented inthe present invention, but may be outside the alloy composition range.In these cases, it could be seen that the condition of the[H]_(F)/[H]_(TM+B+γ), the condition of the balance (TS×E1) of tensilestrength and elongation, the condition of the balance (TS²×HER^(1/2)) oftensile strength and hole expansion, and the condition of bendability(R/t) of the present invention are not all satisfied. Meanwhile, itcould be seen that, in specimen 42, when the total content of aluminum(Al) and silicon (Si) is less than 1.0%, the conditions of[H]_(F)/[H]_(TM+B+γ), the balance (TS×E1) of tensile strength andelongation, and the bendability (R/t) are not satisfied.

While the present invention has been described in detail throughexemplary embodiment, other types of exemplary embodiments are alsopossible. Therefore, the technical spirit and scope of the claims setforth below are not limited to exemplary embodiments.

1. A high-strength steel sheet having superior workability, comprising:by wt %, C: 0.25 to 0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 5.0%or less, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, a balanceof Fe, and unavoidable impurities; and as microstructures, ferrite whichis a soft structure, and tempered martensite, bainite, and retainedaustenite which are hard structures, wherein the high-strength steelsheet satisfies the following [Relational Expression 1]0.4≤[H]_(F)/[H]_(TM+B+γ)≤0.9  [Relational Expression 1] in the aboveRelational Expression 1, [H]_(F) and [H]_(TM+B+γ) are nanohardnessvalues measured using a nanoindenter, [H]_(F) is an average nanohardnessvalue Hv of the ferrite which is the soft structure, [H]_(TM+B+γ) is theaverage nanohardness value Hv of the tempered martensite, the bainite,and the retained austenite which are the hard structures.
 2. Thehigh-strength steel sheet of claim 1, including one or more of thefollowing (1) to (9): (1) one or more of Ti: 0 to 0.5%, Nb: 0 to 0.5%,and V: 0 to 0.5%; (2) one or more of Cr: 0 to 3.0% and Mo: 0 to 3.0%;(3) one or more of Cu: 0 to 4.5% and Ni: 0 to 4.5%; (4) B: 0 to 0.005%;(5) one or more of Ca: 0 to 0.05%, REM: 0 to 0.05% excluding Y, and Mg:0 to 0.05%; (6) one or more of W: 0 to 0.5% and Zr: 0 to 0.5%; (7) oneor more of Sb: 0 to 0.5% and Sn: 0 to 0.5%; (8) one or more of Y: 0 to0.2% and Hf: 0 to 0.2%; and (9) Co: 0 to 1.5%.
 3. The high-strengthsteel sheet of claim 1, wherein a total content (Si+Al) of Si and Al is1.0 to 6.0 wt %.
 4. The high-strength steel sheet of claim 1, whereinthe microstructure of the steel sheet includes, by volume fraction, 30to 70% of tempered martensite, 10 to 45% of bainite, 10 to 40% ofretained austenite, 3 to 20% of ferrite, and unavoidable microstructurecomponents.
 5. The high-strength steel sheet of claim 1, wherein abalance B_(T·E) of tensile strength and elongation expressed by thefollowing [Relational Expression 2] is 22,000 (MPa %) or more, a balanceB_(T·H) of tensile strength and a hole expansion ratio expressed by thefollowing [Relational Expression 3] is 7*10⁶ (MPa²%^(1/2)) or more, andbendability B_(R) expressed by the following [Relational Expression 4]is 0.5 to 3.0,B _(T·E)=[Tensile Strength(TS,MPa)]*[Elongation(El,%)]  [RelationalExpression 2]B _(T·H)=[Tensile Strength(TS,MPa)]²*[Hole ExpansionRatio(HER,%)]^(1/2)  [Relational Expression 3]B _(R) =R/t  [Relational Expression 4] where R is a minimum bendingradius (mm) at which cracks do not occur after a 90° bending test, and tis a thickness (mm) of the steel sheet.
 6. A manufacturing method forhigh-strength steel sheet having superior workability, comprising:heating and hot rolling a steel slab including, by wt %, C: 0.25 to0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 5.0% or less, P: 0.15% orless, S: 0.03% or less, N: 0.03% or less, a balance of Fe, andunavoidable impurities; coiling the hot-rolled steel sheet; coiling upthe hot-rolled steel sheet into a coil; performing a hot-rolledannealing heat treatment on the coiled steel sheet in a temperaturewithin a range of 650 to 850° C. for 600 to 1700 seconds; cold rollingthe hot-rolled annealing heat-treated steel sheet; heating (primaryheating) the cold-rolled steel sheet to a temperature within a range ofAc1 or higher and less than Ac3, and maintaining (primary maintaining)the cold-rolled steel sheet for 50 seconds or more; cooling (primarycooling) the primarily heated steel sheet to a temperature within arange of 100 to 300° C. at an average cooling rate of 1° C./s or more;heating (secondary heating) the primarily cooled steel sheet to atemperature within a range of 300 to 500° C. at an average temperatureincrease rate of 5° C./s or more, and maintaining (secondarilymaintaining) the primarily cooled steel sheet for 50 seconds or more;and cooling (secondarily cooling) the secondarily heated steel sheet toroom temperature.
 7. The manufacturing method of claim 6, wherein thesteel slab further includes one or more of the following (1) to (9): (1)one or more of Ti: 0 to 0.5%, Nb: 0 to 0.5%, and V: 0 to 0.5%; (2) oneor more of Cr: 0 to 3.0% and Mo: 0 to 3.0%; (3) one or more of Cu: 0 to4.5% and Ni: 0 to 4.5%; (4) B: 0 to 0.005%; (5) one or more of Ca: 0 to0.05%, REM: 0 to 0.05% excluding Y, and Mg: 0 to 0.05%; (6) one or moreof W: 0 to 0.5% and Zr: 0 to 0.5%; (7) one or more of Sb: 0 to 0.5% andSn: 0 to 0.5%; (8) one or more of Y: 0 to 0.2% and Hf: 0 to 0.2%; and(9) Co: 0 to 1.5%.
 8. The manufacturing method of claim 6, wherein atotal content (Si+Al) of Si and Al included in the steel slab is 1.0 to6.0 wt %.
 9. The manufacturing method of claim 6, wherein the steel slabis heated to a temperature within a range of 1000 to 1350° C., and issubjected to finish hot rolling in a temperature within a range of 800to 1000° C.
 10. The manufacturing method of claim 6, wherein thehot-rolled steel sheet is coiled at a temperature within a range of 300to 600° C.
 11. The manufacturing method of claim 6, wherein a reductionratio of the cold rolling is 30 to 90%.
 12. The manufacturing method ofclaim 6, wherein a cooling rate of the secondary cooling is 1° C./s ormore.